The 50 Hz, 625 line PAL and SECAM television systems were developed some forty years ago. Those systems have a 50 Hz field rate (312.5 lines per interlaced field) and a 25 Hz frame rate (625 interlaced lines per frame, only 576 of which are visible). At the time of their development, technology did not support large display sizes. Consequently, 576 lines per frame were adequate to render the line structure unnoticeable at normal viewing distances. Also, at that time, display brightness was much lower than in typical displays of the present era. As a result, flicker resulting from the relatively low 50 Hz field rate (refresh rate) was not a serious problem. When PAL and SECAM signals are displayed on today's large screen television equipment, both flicker and line structure are visible and annoying to most viewers.
The problem of flicker is particularly objectionable in 50 Hz systems displayed on large bright screens. To overcome flicker, video systems have been produced that double the frame rate. However, frame doubling still leaves the line structure visible in large displays. Visible line structure is reduced by line doubling. One type of line doubling converts the interlaced signal to a progressively scanned one in which the progressively scanned frame rate is the same as the interlaced field rate and the progressive frames have twice as many lines as an interlaced field. Another type of line doubling maintains interlacing but doubles the number of lines in each interlaced field.
Some “high end” television display enhancement products include both line doubling and frame doubling, including products manufactured by Faroudja Laboratories, Inc. of Sunnyvale, Calif. While providing an excellent picture display without flicker and visible line structure at normal viewing distances, such systems require a high horizontal scan rate. The Faroudja Laboratories product that provides a line doubled and frame doubled progressively scanned output requires a 63 kHz horizontal scan rate, a rate well above the performance capability of all by the best quality and most expensive display systems marketed in relatively small quantities. For the general market, television set manufacturers would like for horizontal scan rates to be below 40 kHz and preferably around 31 kHz for low cost display manufacturing. Thus, a line doubled and frame doubled combination would not be viable in mass market consumer applications. It would be desirable to reduce flicker and perceived line structure, while not requiring an increase in the horizontal scan rate above that supportable by mass market horizontal scanning systems.